Is there any possibility that severe depression can go away by itself without any clinical treatment?

Possible not likely: Depression can resolve on it's own, but i find this rarer and there must be a good support network in place. There are too many good medications to help with the treatment of depression and therapy helps as well. If it is for you, or if it is someone you know, get help now. Do take the risk and don't suffer needlessly.

Yes: Most episodes of depression will diminish over time even without treatment. However, the prolonged suffering of untreated depression, and the risk of serious complications, medical, psychological, and social, suggest that forgoing treatment is not generally wise.